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Gland sealing steam

Gland sealing steam

Gland sealing steam

(OP)
Hi,
Could someone describe in reasonable detail why a high pressure steam turbine needs gland sealing steam supply?

Drex

RE: Gland sealing steam

The gland becomes self-sealing once the internal steam pressure is sufficiently high. Initially there is no internal steam pressure and the external gland steam supply is required to prevent air ingress at the glands and thus allow condenser vacuum to be established prior to rolling the unit.

A typical control scheme may have a supply valve regulating steam from an external source during startup which is closed during normal running, and also a back pressure regulator letting down to the gland steam condenser controlling gland pressure during normal operation.

RE: Gland sealing steam

Here is a typical control scheme that was in a recent post. only shows the header, not the turbine.

it shows the SSFV being the supply of main steam for startup. As load is applied to the turbine, the internal pressure at the HP seals will start providing steam to the header and the SSFV closes. at the load goes higher, the feed from the HP seals is more than required for the LP seals and the SPUV will dump the header to maintan its pressure
http://files.engineering.com/getfile.aspx?folder=6...

RE: Gland sealing steam

(OP)
ScottyUK, I don't follow. You mean at startup (without steam admission to turbine) while establishing condenser vacuum, the turbine is also emptied from air, e.g. you are pulling a vacuum in the turbine and air would leak into the condenser through the turbine. Instead you supply gland steam which leaks in and is condensible. Correct? If so yes I would understand it for the low pressure turbine.

But the high pressure turbine is not connected directly to the condenser. Are you saying it is the case that you draw a vacuum in the high pressure turbine also during start-up and thus the same argument holds as for the LP-turbine? Then once the pressure in the glands is above atmospheric the steam will leak towards atmosphere and there is no need to supply gland steam?

RE: Gland sealing steam

Drexl,
what is your turbine configuration, ie
Does your turbine have reheat?
if yes, does your turbine have an HP and IP turbine or are they combined?
how many LP turbines?

If you know the OEM and turbine Code, they might identify

RE: Gland sealing steam

Yes, you're correct that without steam flow the HP turbine casing is also under condenser vacuum, unless there are other factors involved. Any gland capable of leaking air needs sealing steam during startup. If you can post a copy of the P&I'D we can probably talk you through it. I might have a copy at home, currently visiting family so will be a few days until I have a look.

RE: Gland sealing steam

(OP)
byrdj, question is general but in my current case the setup is HP Turbine - Reheater - LP Turbine - Condenser.

Your answer is: Sealing steam is used for the purpose of keeping air out of the turbine - nothing else.

I didn't think that it would be normal to pull vacuum through the reheater and HP-turbine at startup. If that is the typical case, it is in line with the answer above.

However this current HP turbine is also supplied with sealing steam during normal operation when the pressure is >8 bar and there no possibility of air leaking into the turbine - so there must be some other reason also.
It is not possible to supply P&ID or similar. I'm out of office and don't have access to any documentation.

RE: Gland sealing steam

For every steam turbine I have actually seen or drawings off, pull vacuum all the way to the main control/governor valves (main stop valves too)
besides the rotor shafts that exit the turbine, there is sealing steam to the CVs since they do not backseat and if not provided to the SVs that do back seat when reset, there will be vacuum leakage till the unit is reaset and they are opened.

if you look at the illustrations, you will see that the vacuum is pulled on the condensor, but the LP steam path will allow the vacuum to pulled to the crossovers, then the full vacuum as seen at the condensor will be in the exhuast of the reheat turbine. the RH steam path will allow the vacuum to be pulled all the way to the hot heat piping. then the full vacuum as seen at the condensor will be in the reheater section of the boiler. thus the vacuum in the boiler reheat section will pull on the cold reheat piping all the way to the HP turbine exhaust. the steam path of the HP turbine will all vacuum to be pulled all the way to the SV/CVs.

One could make the aurgument that the CIVs will be closed when the unit is tripped and thus the vacuum in the reheat turbine would not be pulled on the reheater section of the boiler, BUT how about all the drains on the HP turbine like for the extraction piping, the CV/SV below seat drains, they are piped to the condensor so they will pull vacuum even with the CIVs tripped closed.

then, as the unit is being rolled to speed, the CIVs are opened, and the amount of steam needed to roll the unit is so small there will be minumn pressure in the HP section that it will be below atomsperic till there is suffficent load on the unit.

Not sure what you are saying...
"However this current HP turbine is also supplied with sealing steam during normal operation when the pressure is >8 bar and there no possibility of air leaking into the turbine - so there must be some other reason also."

RE: Gland sealing steam

(OP)
I'm saying there must be some second reason for supplying sealing steam other than to prevent air leakage. During operation the HP turbine pressure is > 8 bar so the steam must be leaking out from the turbine towards atmosphere = air can't leak in. I'm sure the HP turbine is not supplied with sealing steam by mistake during operation so it must have some function. I would like to understand what this function is.

Regarding your description about pulling vacuum all the way to the HP turbine inlet valves it got through. Thanks for this information!

RE: Gland sealing steam

You are absolutely correct, There is no need to SUPPLY STEAM TO the HP turbine seals once there is pressure inside the HP casing.

Even more so, once the HP casing pressure is above about 1/3 to 1/2 rated, there is no way to supply steam since the pressure trying to escape from the ends will be greater than the sealing pressure. Once the turbine is loading, the steam that would be forced out of the ends of the HP turbine will now flow INTO the steam seal header and the startup steam seal feed valve needs to be closed. But instead of letting the steam blow out of the ends of the HP turbine, that steam can now be used to supply the steam to the LP turbine seals.

so for a design purpose discription, the HP seals are connected to the steam seal header that also provides steam to the LP turbine during startup and low load.
Once at load, the HP seals are connected to the steam seal header so that the waste steam that would blow from the ends can be used to seal the LP turbine and the original main steam used for sealing during startup is no longer needed and used to make MWs. So you are also correct in that there is another function

Given that the design has an uncontrollable supply of sealing steam from the HP turbine at load, the steam seal header MUST have a means to control the pressure to the LP seals. the steam seal controls will close the feed as the pressure increases and then open a DUMP valve to releive the excessive flow from the HP turbine seals. this dump can go directly to the condensor (and thus be wasted) or it can be routed to a low pressure heater.

the link in my first post shows a control scheme for the steam seal header
http://files.engineering.com/getfile.aspx?folder=6...
the startup feed valve - SSFV
the at load dump valve - SPUV

RE: Gland sealing steam

Gland steam systems allow bi-directional flow. When casing pressure is lower than atmospheric, external steam is supplied via a letdown valve and de-superheater. When casing pressure is higher than atmospheric the gland sealing steam passes outward toward the gland steam condenser through a spillover valve. See the attached Westinghouse P&ID for an example of this type of control scheme. Note the flow direction arrows.

RE: Gland sealing steam

Scotty, please clarify WH drawing practices for me.
Is Hex 062 indicating a connection from a steam source and
Hex 063 indication a connection to a steam sink

Are you saying WH uses a seperate condensor for the excessive seal steam when it dumps?

RE: Gland sealing steam

(OP)
To sum it up:
-We are supplying external sealing steam to HP turbine during operation when the pressure in the turbine is > 8bar
-According to byrdj the function of the sealing steam is to keep out air, so sealing steam is not required once the pressure in the turbine section is above atmospheric

-> Either the plant operation principle is wrong OR I have misunderstood the operating instructions OR byrdj is wrong. I don't know which at the moment.

RE: Gland sealing steam

Assuming you are suppling steam to HP turbine when the internal pressure is >8 bar;

do you provide sealing steam to the HP turbine when it is <8 bar?

Is your sealing steam to the HP turbine seperate than the sealing steam supplied to the LP turbine? Or is the same sealing steam header used for both

what is your sealing steam pressures? how are these pressures controlled?

does your steam seal design use a Gland header (seperate condensor with few inches of water vacuum that draws air and escaping sealing steam?

can you give us the brand name, possible model discription?

RE: Gland sealing steam

Hi byrdj,

Machine is an early 1990s vintage and probably considerably older in design.

Yes, there is a separate gland steam condenser. It's a fairly small tube & shell exchanger fed from cooling water tapped of the supply to the main L/O cooler bank. Hex 062 is a source upstream of the MSV's on the HP steam header. Hex 063 goes to the gland steam condenser. The two valves operate with a split range control scheme so at mid-span both valves are closed. In practice either the supply valve is delivering steam to the gland or the backpressure valve is allowing flow to the G/S condenser. It's a pig of a system to set up accurately unless the positioners are in A-1 condition.

RE: Gland sealing steam

thanks for the education of that "other brand" :)

are you positive hex 063 goes to "gland condensor" and not main condesor or to the last heater?

Given you went to all the effort to post a WH P&ID, I cleaned up a GE P&ID. (removed unit specific and heat balance data) 1970s early GE EHC.

I wanted to post one of my favorite design, a MHC, but the detail of the representation of the feed/dump controller was reduced to a box

Being an old mechanical/hydraulic type of guy, I though it was going too complex to apply pnematics like the one I'm posting, but I see the WH has signals from DEH! Pretty easy to get the overlaps of the feed/dump when it is just one MHC actuator controlling both. and It is still amazing those MHC could be absolute isobaric where the pnematic has to use a pressure range.

This P&ID is the same control scheme as shown in my first illustration

RE: Gland sealing steam

Scotty
is the grammer of my narritive discribing the transition from HP feed to HP bleed comprehensiable?

RE: Gland sealing steam

(OP)
Assuming you are suppling steam to HP turbine when the internal pressure is >8 bar;

do you provide sealing steam to the HP turbine when it is <8 bar? YES (both during start-up and operation)

Is your sealing steam to the HP turbine seperate than the sealing steam supplied to the LP turbine? Or is the same sealing steam header used for both Separate. There are two systems one LP and one HP sealing steam systems/headers

what is your sealing steam pressures? how are these pressures controlled? HP-header about 8,5 bar (HP sealing steam control valve take live steam and lower the pressure to HP-turbine outlet pressure + 0,5 bar) and LP-header a bit over atmospheric (controlled by a external supply valve during start-up. When pressure in the turbine is high enough for it to supply itself with gland steam, the external supply valve closes

does your steam seal design use a Gland header (seperate condensor with few inches of water vacuum that draws air and escaping sealing steam? Yes

can you give us the brand name, possible model discription? Don't have documentation available at the moment

RE: Gland sealing steam

By having seperate HP turbine and LP turbine steam sealing systems/headers, your system is unlike any I have ever seen. Thus the reason for confussion.

does the LP turbine system get its start up feed from main steam, same as HP turbine system?

If the LP turbine system becomes self substained at load and the feed valve closes, then at least one end of the LP turbine must have steam pressure agaisnt the gland seal. I have been thinking both ends of the LP turbine gland seal will only see condensor vacuum.

Is this so, that one end of your LP turbine has steam pressure that varies with load agaisnt the seal? would the LP turbine be a single flow within a seperate casing/hood?

Is there a valve that dumps the LP turbine sealing steam to the condensor if the gland leakage was to exceed flow need for being self substained?

If the LP turbine system can become self substained with load, I would think the HP turbine would do the same.

does the HP turbine steam seal header pressure stay about 0.5bar from startup to full load?

Is there a valve that dumps the HP turbine sealing steam if it were to try to become self substained and its feed valve remained open?

Are we confussion HP and LP turbine sealing systems with the fact that a single steam seal system has two headers, the sealing header with is low pressure, pure steam and the gland exhaust header with is just a few inches of water LESS than atmosphiric and is a mix of sealing steam and air that ingresses at the turbine ends going to the gland exhauster condensor?

we difinately need to be see actual P&IDs since we are own completely different pages.

RE: Gland sealing steam

Yes, Hex 63 is definitely a separate gland steam condenser. One of the last big outages I was involved with on that set had a tormented return-to-service and we couldn't establish vacuum. The G/S condenser was choked with about 200lb of sandy debris, which was finally discovered after every instrument on the whole machine was blamed ahead of it.

The 'signals from the DEH' are just a single 4-20mA loop, and the two I/P positioners are calibrated so that at mid-span (12mA) both valves are closed. Movement below 12mA opens one, above 12mA opens the other. A single actuator and common linkage would have been a far better solution.

The DEH was a good system, more capable than most of that era.

I can follow your GE terminology and translate it with a little effort. tongue Yes, all nice and clear. We need a P&ID because drexl appears to have something slightly unusual - wonder if it is an Eastern European design?

Found this on Youtube, not sure if it will help drexl: http://www.youtube.com/watch?v=xKtpL5DB7LY

RE: Gland sealing steam

Thanks for ensuring my narritive makes sense...
I too have search, looking for a system like Drexl appears to be discribing. I found that youtube. that system appears to be similair to what each of us have presented, except the vidio's narrative is either misleading or just incomplete. I noticed at 0:54 to 0:58 there is a connection to the supply piping teeing off to a control valve that then is piped to the condensor, thus the header unloader valve for when the HP leak off becomes excessive for LP sealing. then at 1:01 to 1:04, you can notice that sealing steam is going towards the HP seal

that HP leak off that goes accross the top does not appear to used by the seal, but just a means to utilize some of HP leak off for making MWs and reduce the HP leakoff going to feed the seal steam header.

Durring searching, I did notice a "patent" drawing that look like it had seperate pressure regulating valves for the LP and HP seal, but the link to a readable drawing was dead.

"Tuning" the pnumatic controllers on the GE system was pure Voodo. 20 years ago, we came up with settings that were suppose to be suppleid in GEKs. without that info from the factory or able to find my old notes, it becomes the most difficult challange to a new unit since you need the operating extreme conditions to gather response and asking for a full load trip for testing is not appreacaite by any utility. where with the old MHC regulator, all the controlling engineer (lead/lag/gains & overlap) was bult into it, just get the linkage in good order and it worked

I am very curoius about Drexls system and hope we can see a P&ID. it is still nice to learn

RE: Gland sealing steam

byrdj,

You got me looking more closely and Hex 063 goes to the MAIN condenser. It has been a few years since I was on this unit and my memory is terrible. blush The gland steam condenser itself is shown on the P&ID I posted. Sorry for the error.

You studied that animation in some detail, I missed those inconsistencies that you spotted.

I love the little things that the OEMs leave for the field guys to figure out. WH were no better, we had plenty of little notebooks and crib sheets filled with critical information which the OEM didn't share with us; I don't think we were unique in that respect by any means.

RE: Gland sealing steam

Drexls, any chance to see your P&ID?

RE: Gland sealing steam

One reason to pull vacuum with sealing system on in the whole train (HP, RH and LP) is to minimize windage losses, potential overheating of the buckets and rubs.
The sealing system serves two purposes: avoiding ingress of air and exit of steam

saludos.
a.

RE: Gland sealing steam

QUOTE
------------------
Are we confussion HP and LP turbine sealing systems with the fact that a single steam seal system has two headers, the sealing header with is low pressure, pure steam and the gland exhaust header with is just a few inches of water LESS than atmosphiric and is a mix of sealing steam and air that ingresses at the turbine ends going to the gland exhauster condensor?
------------------

I marked up an illustration that shows a HP and LP turbine end packing with the sealing and gland exhaust headers.
It is shown with a HP stage pressure that is being leaked off into the steam seal header.

this can be combined with the illustration of the seal steam header controls that shows the SSH being supplied by main steam during startup, then transitioning to HP stage leakoff as the turbine is loaded. thus the SSH feed valve will close as turbine is loaded.

the gland exhauster header is allows maintained at a slightly below atmosperic pressure so that it prevents steam from escaping the turbine and since this steam is mixed with room air, it is taken to the gland exhauster condensor.

RE: Gland sealing steam

(OP)
Hi,
I gathered some more informaation and the issue seems to be solved now. Unfortunately I can not give a P&ID but this is not required to understand what the function of the higher pressure steam is.

The reason why the HP steam is supplied to the HP-turbine has nothing with air to do. Instead it is supplied so that the steam from the turbine will not leak through the labyrinth - instead this HP-steam will leak into the turbine. The reason is that the steam leaving the HP-turbine is wet, and the steam would cause erosion in the labyrinths. The HP-steam is instead dry with some superheating and no erosion occurs.

Thanks for all the good answers!

RE: Gland sealing steam

Thanks...That makes perfect sense ...

ScottyUK, I appreaciate the information related to WH. Are you familair with the IGTC site?

RE: Gland sealing steam

byrdj,

No hadn't seen that one but have just had a little look. You might see a familiar name appear there if they let me in. winky smile

No problem with the information from Big W, I have a reasonable amount of info on that particular set and some fairly detailed knowledge of the DEH control system and the generator & AVR.

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